U.S. patent number 8,228,836 [Application Number 12/479,563] was granted by the patent office on 2012-07-24 for cooperative mac for rate adaptive randomized distributed space-time coding.
This patent grant is currently assigned to Polytechnic Institute of New York University. Invention is credited to Elza Erkip, Thanasis Korakis, Pei Liu, Shivendra S. Panwar, Anna Scaglione.
United States Patent |
8,228,836 |
Erkip , et al. |
July 24, 2012 |
Cooperative MAC for rate adaptive randomized distributed space-time
coding
Abstract
Data is transmitted from a source wireless device to a
destination wireless device by: (a) discovering node-to-node
wireless channel conditions in a wireless network; (b) determining
at least one of (A) wireless relay devices, (B) modulation schemes,
and (C) transmission rates using the discovered node-to-node
channel conditions; (c) signaling at least some of the determined
information to the determined wireless relay devices; (d)
receiving, with each of the wireless relay devices, a transmission
of the data from the source wireless device; and (e) transmitting,
with each of the wireless relay devices, a randomized, space-time
encoded, part of the received data, to the destination device using
the signaled at least some of the determined information.
Inventors: |
Erkip; Elza (New York, NY),
Korakis; Thanasis (Brooklyn, NY), Liu; Pei (Forest
Hills, NY), Panwar; Shivendra S. (Freehold, NJ),
Scaglione; Anna (Davis, CA) |
Assignee: |
Polytechnic Institute of New York
University (Brooklyn, NY)
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Family
ID: |
41530228 |
Appl.
No.: |
12/479,563 |
Filed: |
June 5, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100014453 A1 |
Jan 21, 2010 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61059032 |
Jun 5, 2008 |
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Current U.S.
Class: |
370/315 |
Current CPC
Class: |
H04B
7/2606 (20130101); H04B 7/15592 (20130101); H04L
1/0003 (20130101); H04L 1/0625 (20130101); H04B
7/026 (20130101); H04L 5/0023 (20130101); H04L
2001/0097 (20130101) |
Current International
Class: |
H04B
7/14 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 2007/140437 |
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Dec 2007 |
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WO |
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Primary Examiner: Ly; Anh-Vu
Attorney, Agent or Firm: Pokotylo; John C. Straub &
Pokotylo
Government Interests
.sctn.0.0 GOVERNMENT RIGHTS
The United States Government may have certain rights in this
invention pursuant to a grant awarded by the National Science
Foundation. Specifically, the United States Government may have a
paid-up license in this invention and the right in limited
circumstances to require the patent owner to license others on
reasonable terms as provided for by the terms of Award No. 0520054
awarded by the National Science Foundation (Division of Computer
and Network Systems).
Parent Case Text
.sctn.0.1 RELATED APPLICATIONS
Benefit is claimed to the filing date of U.S. Provisional Patent
Application Ser. No. 61/059,032 ("the '032 provisional"), titled
"COOPERATIVE MAC FOR RATE ADAPTIVE RANDOMIZED DISTRIBUTED
SPACE-TIME CODING," filed on Jun. 5, 2008 and listing Elza ERKIP,
Thanasis KORAKIS, Pei LIU, Shivendra S. PANWAR and Anna SCAGLIONE
as inventors. The '032 provisional is incorporated herein by
reference. However, the scope of the claimed invention is not
limited by any requirements of any specific embodiments described
in the '032 provisional.
Claims
What is claimed is:
1. A method for transmitting data from a source wireless device to
a destination wireless device, the method comprising: a)
discovering device-to-device wireless channel conditions in a
wireless network including the source wireless device; b)
determining, using the discovered device-to-device channel
conditions, both (1) an estimated number of wireless relay devices
to be used, cooperatively, to forward data from the source wireless
device to the destination wireless device, and (2) at least one of
(A) a modulation scheme to be used by the wireless relay devices to
forward data from the source wireless device to the destination
wireless device, or (B) a transmission rate to be used to by the
wireless relay devices to forward data from the source wireless
device to the destination wireless device; c) signaling to a
plurality of wireless relay devices, randomized, distributed
space-time coding control parameters, wherein the randomized,
distributed space-time coding control parameters are derived from
the determined estimated number of wireless relay devices to be
used, cooperatively, to forward data from the source wireless
device to the destination wireless device, and (2) at least one of
(A) a modulation scheme to be used by the wireless relay devices to
forward data from the source wireless device to the destination
wireless device, or (B) a transmission rate to be used to by the
wireless relay devices to forward data from the source wireless
device to the destination wireless device; d) receiving, with each
of the plurality of wireless relay devices, a transmission of the
data from the source wireless device; and e) transmitting, with
each of the plurality of wireless relay devices, a randomized,
space-time encoded, part of the received data, to the destination
device using the signaled randomized, distributed space-time coding
control parameters.
2. A system for transmitting data from a source wireless device to
a destination wireless device, the system comprising a plurality of
relay devices, the system including: a) means for discovering
device-to-device wireless channel conditions in a wireless network
including the source wireless device; b) means for determining,
using the discovered device-to-device channel conditions, both (1)
an estimated number of wireless relay devices to be used,
cooperatively, to forward data from the source wireless device to
the destination wireless device, and (2) at least one of (A) a
modulation scheme to be used by the wireless relay devices to
forward data from the source wireless device to the destination
wireless device, or (B) a transmission rate to be used to by the
wireless relay devices to forward data from the source wireless
device to the destination wireless device c) means for signaling to
the plurality of wireless relay devices, randomized, distributed
space-time coding control parameters, wherein the randomized,
distributed space-time coding control parameters are derived from
the determined estimated number of wireless relay devices to be
used, cooperatively, to forward data from the source wireless
device to the destination wireless device, and (2) at least one of
(A) a modulation scheme to be used by the wireless relay devices to
forward data from the source wireless device to the destination
wireless device, or (B) a transmission rate to be used to by the
wireless relay devices to forward data from the source wireless
device to the destination wireless device; d) means for receiving,
with each of the plurality the plurality of wireless relay devices,
a transmission of the data from the source wireless device; and e)
means for transmitting, with each of the plurality of wireless
relay devices, a randomized, space-time encoded, part of the
received data, to the destination device using the signaled
randomized, distributed space-time coding control parameters.
Description
.sctn.1. BACKGROUND OF THE INVENTION
.sctn.1.1 Field of the Invention
The present invention concerns wireless communications. In
particular, the present invention concerns cooperative wireless
communications used to improve data throughput, reliability, and/or
range.
.sctn.1.2 Background Information
In a typical wireless network, each transmitter is surrounded by
several other stations (or more generally, nodes or devices).
However, usually, only the link between the sender device and the
receiver device is used to send the data. In a fading environment,
data transmission over this link might not be reliable. For
example, deep fading and interference may cause signal corruption,
which results in the data getting lost.
Cooperative wireless communication techniques provide a potential
solution by recruiting relays or helpers. When a relay is employed,
the possibility of losing data or receiving inaccurate data
decreases. (See, e.g., the articles: A. Sendonaris, E. Erkip, and
B. Aazhang, "User Cooperation Diversity-Part I: System
Description," IEEE Trans. on Communications, Vol. 51, No. 11, pp.
1927-1938 (November 2003) (incorporated herein by reference); A.
Sendonaris, E. Erkip, and B. Aazhang, "User Cooperation
Diversity--Part II: Implementation Aspects and Performance
Analysis," IEEE Trans. on Communications, Vol. 51, No. 11, pp.
1939-1948 (November 2003) (incorporated herein by reference); J. N.
Laneman, G. W. Wornell, and D. N. C. Tse, "An Efficient Protocol
for Realizing Cooperative Diversity In Wireless Networks," ISIT
2001, p. 294 (June 2001) (incorporated herein by reference); and J.
N. Laneman, D. Tse, and G. Wornell, "Cooperative Diversity In
Wireless Networks: Efficient Protocols and Outage Behavior," IEEE
Transactions on Information Theory, Vol. 50, No. 12, (December
2004) (incorporated herein by reference).)
While cooperative communications is actively researched in the
physical layer ("PHY"), modifications to the higher layer protocol
stack are needed to discover and utilize all relays. Previous work
by at least some of the present inventors presented a cooperative
MAC protocol for IEEE 802.11. (See, e.g., the references: P. Liu,
Z. Tao, and S. Panwar. "A Cooperative MAC Protocol for Wireless
Local Area Networks," IEEE Intl. Conf. on Communications (Seoul,
Korea, June 2005) (incorporated herein by reference); T. Korakis,
Z. Tao, Y. Slutskiy, and S. Panwar, "A Cooperative MAC Protocol for
Ad-Hoc Wireless Networks", PWN07, (White Plains, N.Y., March 2007)
(incorporated herein by reference); and S. Panwar, P. Liu and Z.
Tao, "Cooperative Wireless Communications", U.S. Pat. No. 7,330,457
(incorporated herein by reference).) Each packet is transmitted
over two hops, first from the source to the relay and then from the
relay to destination. These references demonstrated that the
network performance metric, such as throughput and delay
performance can be greatly improved by using cooperation.
Typically, there might be more than one device (or node) that can
"overhear" a packet sent by a source device. If such devices are
willing and able to transmit cooperatively to the destination
device, cooperative diversity can be much larger than in the case
of a single relay. However, if all relay devices transmit
sequentially in time, the time required to complete transmission
increases linearly with the number of relay devices. Thus, although
using more relay devices increases diversity, network throughput,
which is measured by the number of bits successfully received in a
unit time, may actually decrease when the system employs a lot of
relay devices.
Recent advances in Multi-Input Multi-Output ("MIMO") systems allow
multiple antennas to transmit together to achieve high diversity
gains using space-time coding ("STC"). Hence, multiple relay
devices may be used as a distributed antenna array to mimic a MIMO
system. In fact, the spatial diversity obtained by cooperation
increases linearly with the number of relays. Simultaneous
transmission of multiple relay devices that decode the source
information at the PHY layer can be accomplished by a distributed
space-time code ("DSTC"). (See, e.g., J. N. Laneman, D. Tse, and G.
Wornell, "Cooperative Diversity In Wireless Networks: Efficient
Protocols and Outage Behavior," IEEE Transactions on Information
Theory, Vol. 50, No. 12, (December 2004) (incorporated herein by
reference).) FIG. 1 illustrates such a transmission. The basic idea
is to coordinate and synchronize the relay devices so that each
relay device acts as one antenna of a regular STC. (See, e.g., the
articles: V. Tarokh, H. Jafarkhani, and A. Calderbank, "Space-Time
Block Codes from Orthogonal Designs," IEEE Transactions on
Information Theory, Vol. 45, No. 5, pp. 1456-1467 (July 1999)
(incorporated herein by reference); and S. Alamouti, "A Simple
Transmit Diversity Technique for Wireless Communications," IEEE
Journal on Selected Areas in Communications, Vol. 16, No. 8, pp.
1451-1458 (October 1998) (incorporated herein by reference).)
Unfortunately, however, DSTC poses a number of challenges from a
system perspective. Each relay participating in a DSTC needs to be
assigned a number (and each relay must know of its number) so that
it knows exactly which antenna it will mimic in the underlying STC.
Further, DSTC does not fully exploit all available relay devices
because even though stations other than the chosen relay devices
may decode the source information correctly, they are not allowed
to transmit. This sacrifices potential diversity and coding gains.
Also, each individual link between the source and the relay might
not be reliable due to noise, interference, and/or mobility. If one
of the relay devices does not receive the signal from the source
device, it cannot forward it in the next hop, which degrades the
diversity gain of the system.
The application of DSTC communications in mobile ad hoc networks
has been discussed in the article, G. Jakllari, S. V.
Krishnamurthy, M. Faloutsos, P. V. Krishnamurthy, and O. Ercetin,
"A Framework for Distributed Spatial-Temporal Communications in
Mobile Ad hoc Networks," IEEE INFOCOM, (Barcelona, Spain, April
2006) (incorporated herein by reference).
In that article, the source device can only recruit a fixed number
of relay devices. The source device transmits a packet in the first
hop and expects a busy tone signal from each of the relay devices
sent sequentially in time. Even if a single relay device does not
receive the packet from the source device, and consequently fails
to respond with a busy tone, the source device transmits directly
to the destination device rather than rely on the recruited relay
devices.
Thus, it would be useful to improve the known DSTC systems.
.sctn.2. SUMMARY OF THE INVENTION
Exemplary embodiments consistent with the present invention provide
a layer-2 protocol to (1) discover node-to-node channel conditions
in a wireless network, (2) determine values for randomized
distributed space-time coding ("R-DSTC") techniques which alleviate
at least some of the foregoing problems of known DSTC systems, and
(3) signaling the determined values to relay devices. Each relay
device transmits a random, linear combination of antenna
waveforms.
Thus, at least some exemplary embodiments consistent with the
present invention provide a MAC (layer 2--data link) protocol to
support R-DSTC. Such exemplary embodiments might provide a
signaling protocol which enables discovery of neighboring relays
for a generic wireless network (including infrastructure based
networks and mobile ad hoc networks). A source device can then
"recruit" relay devices, as needed. The recruited relay devices can
then relay, cooperatively, information received from the source
device to the destination device. The recruited relay devices may
do so using R-DSTC and a modulation scheme assigned by the source
device.
At least some exemplary embodiments consistent with the present
invention may adapt the transmission rate. Such rate adaptation
might use at least one of (A) the number of relay devices
available, (B) a more complete estimate of network information, and
(C) destination device receiver feedback. In a dense network, exact
knowledge of channel conditions is not required for R-DSTC to reach
its full potential.
.sctn.3. BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an exemplary environment in which embodiments
consistent with the present invention may operate.
FIG. 2 is a block diagram of an exemplary relay device architecture
consistent with the present invention.
FIG. 3 includes flow diagrams of exemplary operating methods,
consistent with the present invention, of a source device, each of
a plurality of relay devices, and a destination device.
FIG. 4 is a flow diagram of an exemplary MAC layer protocol
consistent with the present invention.
FIG. 5 illustrates simulations of how network throughput depends on
the number of relay devices under various transmission
techniques.
FIG. 6 illustrates simulations of how service delay depends on the
number of relay devices under various transmission techniques.
.sctn.4. DETAILED DESCRIPTION
.sctn.4.1 Exemplary Environment in Which Embodiments Consistent
with the Present Invention May Operate
FIG. 1 illustrates an exemplary environment 100 in which
embodiments consistent with the present invention may operate. As
shown, the environment 100 includes a source device 110, a
plurality of (e.g., peer) relay devices 120 and a destination
device 130. Assume that the destination device 130 includes a
plurality of (L) antennas.
As shown by lines 140, to maximize spatial multiplexing gain, the
source device 110 first broadcasts a data packet (or more
generally, an information stream) for receipt by each relay device
120. Then, as shown by lines 150, the relay devices 120 transmit
cooperatively to the destination device 130 in parallel, thereby
achieving a high spatial multiplexing gain.
.sctn.4.2 Exemplary Relay Architecture
FIG. 2 is a block diagram illustrating an architecture of an
exemplary relay device 200 consistent with the present invention.
The exemplary relay device 200 may include a transmit/receive
antenna 210, a receiver portion 220 and a transmitter portion
230.
The receiver portion 220 includes a single-input-single-output
("SISO") receiver 225 which accepts a signal received by antenna
210 (from source device), and which outputs a (e.g., serial) signal
to the transmitter 230.
The transmitter portion 230 includes a space-time processor 240
followed by a randomizing processor 250. The space-time processor
240 may be a known STC processor and may use known STC processing
techniques. On the other hand, the use of the randomizing processor
250 in the context illustrated is new and advantageous.
The space-time processor includes a channel encoder 242 and a
space-time encoder 246. Basically, the serial output signal of the
SISO receiver is encoded by the channel encoder 242 to generate an
encoded signal. The space-time encoder 246 accepts the encoded
signal and outputs a plurality of (L) signals (X.sub.1 through
X.sub.L).
The relay device 200 may include a device for generating a random
vector of length L, where each component of the random vector of
the i.sup.th relay device 200 is denoted as R.sub.i,1 through
R.sub.i,L as shown. Each of the L outputs of the space-time encoder
246 is mixed with each component of the random vector with mixers
252. The L outputs of the L mixers 252 are then combined by
combiner 254 to obtain an output signal (Z.sub.i). The output
signal Z.sub.i is then transmitted for reception by the destination
device. Each of the plurality of relay devices will therefore
transmit, simultaneously (or effective simultaneously), a random
linear combination of all the L streams. Note that if the received
information stream is B bits, the output signal Z.sub.i will be B/L
bits.
Note that FIG. 2 only depicts the signal processing providing
relaying functionality. In principle, all devices can act as source
device, relay device, and/or destination device, as needed, for
each packet. That is, dedicated relay devices and separate hardware
architecture for the relay devices are not required.
At least some embodiments consistent with the present invention may
be implemented in hardware (e.g., integrated circuits, application
specific integrated circuits, programmable logic or gate arrays,
etc.), and/or software (e.g., program instructions stored in memory
such as a RAM, ROM, etc., and/or stored on a storage device such as
a magnetic or optical disk, etc., executed on a general purpose
processor such as a microprocessor).
.sctn.4.3 Exemplary Methods
FIG. 3 includes flow diagrams of exemplary operating methods 310,
320, and 360, consistent with the present invention, of a source
device, each of a plurality of relay devices, and of a destination
device, respectively. Each packet (or more generally, information
stream) transmission from source device to destination device
occurs in two steps. Referring to the left side of FIG. 3, in the
first step, the source device broadcasts its packet (or information
stream) for reception by its (peer) relay devices. (Block 315)
Referring back to blocks 315 and 325, in at least some embodiments
consistent with the present invention, the source device may append
a frame checksum ("FCS") to the sourced packet. In such a case,
each relay device receiving the packet first tries to decode the
packet and verify the FCS to see if it received the packet
correctly. Each relay device will participate in the cooperative
transmission only if it receives correct information.
Referring now to the middle of FIG. 3, in the second step, M
cooperative relay devices (acting as virtual antennas) transmit
together in parallel and in synchronization. Each of the relay
devices might include a transceiver architecture such as described
above with reference to FIG. 2. Referring to FIG. 3, upon receipt
of the packet (or information stream) (event 325) each relay device
encodes (e.g., with a channel encoder) the received packet or
information stream (Block 330). Space-time encoding is then applied
to the encoded stream to generate L parallel streams (Block 335).
(Recall 246 of FIG. 2.)
Instead of letting each relay device pick a unique data stream from
the output of the coordinate system (as is the case with known
systems), the i.sup.th relay device independently generates a
random vector r.sub.i of length L (Block 340) and generates a
linear combination of the L data streams using the random vector to
form an output signal (Block 345). (Recall, e.g., 250 of FIG. 2.)
The method 320 then transmits the output signal to the destination
device. (Block 350)
Referring now to the right side of FIG. 3, when destination
operations 360 receive output signals from the relays (event 365),
they combine and decode the received signals (Block 370).
As can be appreciated from the foregoing, the cooperative
transmission of a packet takes two time slots as described in the
reference P. Liu, Z. Tao, and S. Panwar. "A Cooperative MAC
Protocol for Wireless Local Area Networks," IEEE Intl. Conf. on
Communications (Seoul, Korea, June 2005) (incorporated herein by
reference). In the first time slot, the source device S first
transmits the packet to all its potential relay devices R. Assuming
each packet is followed by a frame checksum (FCS) code, each relay
device first tries to decode the packet and verify the FCS. In the
following time slot, only the relay(s) that receive the packet
correctly re-encode and send the packet to the destination device.
Thus, the transmission for a packet from a source device to a
destination device takes two hops, as in the reference P. Liu, Z.
Tao, and S. Panwar. "A Cooperative MAC Protocol for Wireless Local
Area Networks," IEEE Intl. Conf on Communications (Seoul, Korea,
June 2005) (incorporated herein by reference). However, in the
R-DSTC scheme consistent with the present invention, multiple relay
devices may transmit simultaneously in the second time slot. The
signals from all relay devices propagate to the destination, where
they are received.
Although not shown in relay operations 320, each relay device may
employ a regular single-input and single-output ("SISO") decoder to
decode the information from the source packet sent in the first
hop. Referring back to block 330, the relay device re-encodes the
information stream received from the source.
Referring back to block 335, the output from the space-time (STC)
encoder is in the form of L parallel streams, each corresponding to
an antenna in a MIMO system with L transmission antennas. Assume an
underlying full rank space-time code G is of size L.times.K. Even
if each relay device only has one antenna (and therefore cannot
transmit all streams in parallel), each of the relay devices
transmits a different weighted sum of all the streams.
Referring back to blocks 340 and 345, for the i.sup.th relay at
time m, the transmitted signal may be expressed as:
z.sub.i(m)=E.sub.s.eta.X(m), where i=1,2, . . . , N and m=1,2, . .
. , K. E.sub.s is the symbol energy and r.sub.i=[r.sub.i1,
r.sub.i2, . . . , r.sub.iL] is the random weight at relay i.
Each element of r.sub.i is assumed to be an independently generated
random variable, such as a complex Gaussian random variable with
zero mean and variance 1/L. (See, for example, the article B. S.
Mergen and A. Scaglione, "Randomized Space-Time Coding for
Distributed Cooperative Communication," IEEE Transactions on Signal
Processing, pp. 5003-5017 (October 2007) (incorporated herein by
reference).) Assume that there are N relays.
X(m)=[x.sub.1(m)x.sub.2(m) . . . x.sub.L(m)].sup.T includes the
coded symbols from the STC encoder and is the m.sup.th column of
the space time code G.
Referring back to 365 and 370 of FIG. 3, the destination receiver
may be similar to a regular STC receiver. Assuming that N nodes
participate in relaying, the received signal at the receiving
antenna at the m.sup.th symbol time can be expressed as:
y(m)=HZ(m)+w(m)=E.sub.sHRX(m)+w(m).
Here, H is the 1.times.N channel vector representing channel gain
from each relay to the destination, Z(m)=[z.sub.1(m)z.sub.2(m) . .
. z.sub.L(m)].sup.T and
##EQU00001##
The additive Gaussian white noise is w(m) and has a power spectrum
density of N.sub.0/2.
Still referring to block 370 of FIG. 3, the receiver at the
destination device requires channel information to achieve optimum
performance using STC. However, in a real system, perfect channel
information is not attainable. The channel is often estimated using
pilot symbols. An interesting discovery is that, for systems
consistent with the present invention, there is no need to estimate
H and R separately; only the effective channel vector G:=HR is
required, based on the article B. S. Mergen and A. Scaglione,
"Randomized Space-Time Coding for Distributed Cooperative
Communication," IEEE Transactions on Signal Processing, pp.
5003-5017 (October 2007) (incorporated herein by reference). By
letting X=I (i.e., by letting each branch of the output of the
space-time encoder transmit a pre-defined symbol sequentially in
time), the received signal would be: Y=E.sub.sHR+W.
The regular receiver channel estimation technique for STC may be
used to estimate the effective channel vector G.
Assume that the channel undergoes independent Rayleigh fading, and
the random coefficient is Gaussian distributed with zero mean and
variance of 1/L. The bit error probability in high SNR region can
be found in the paper P. Liu, Y. Liu, T. Korakis, A. Scaglione, E.
Erkip and S. Panwar, "Cooperative MAC for Rate Adaptive Randomized
Distributed Space-time Coding", IEEE Global Telecomm. Conf. (IEEE
GLOBECOM (New Orleans, November 2008).
A regular space-time decoder can decode the received signal as long
as G is known.
.sctn.4.3.1 Exemplary Rate Adaptation
If it is desired to maximize a network layer performance metric,
such as throughput and delay, the PHY layer operations have to be
coupled with the activities of the MAC layer. Most wireless
networks use rate adaptation to handle a range of receiving SNR at
the receiver to maintain a satisfactory error probability. It is
important for any device with multirate capabilities to use the
channel resource efficiently. One of the criteria for rate
adaptation is to keep the error rate below a pre-set threshold,
while maximizing the throughput for each source device and
destination device pair.
In at least some embodiments consistent with the present invention,
the MAC selects the rates for both transmission hops (the source
device to the relay devices, and the relay devices to the
destination device). This is because the effective throughput
significantly depends on the coding and modulation schemes.
Generally speaking, the higher the data rate for the first hop
transmission, the less time is consumed for the first hop. However,
fewer devices can decode the first transmission and participate in
the second hop as a relay device. Fewer relay devices means the
supported data rate for the second hop is expected to be lower and,
consequently, more time is consumed in the second hop. Therefore,
there is a tradeoff between the data rates of the first and the
second hops to maximize the throughput.
In at least some embodiments consistent with the present invention,
MAC may also be used to choose a suitable STC to be used by the
relay devices. For example, the MAC may also attempt to choose a
STC dimension L as close as possible to the number of relay devices
N to maximize diversity gains. However, practically, good
space-time code only exists for a selected set of all L's. That is,
some numbers work better for space-time coding than others. If L
does not work well for space-time coding, some lower value (e.g.,
L-1) might be used instead.
In at least some embodiments consistent with the present invention,
the PHY layer is designed to handle BPSK, QPSK and different size
QAM constellations. The rate that the PHY layer can support is
denoted as R.sub.p, p=0, . . . , P, where R.sub.0 is the basic rate
at which the devices (nodes) exchange control information. Assume
that there are M devices (nodes) in the network and each of them
has a range such that the basic rate R.sub.0 is decoded with high
probability. Further assume that the packet header is at the basic
rate R.sub.0 and that the received signal strength is available at
the MAC. So each device (node) can estimate the channel conditions
between a pair of devices (nodes) that can communicate.
For each rate R.sub.p let {A.sub.p}.sub.ij=a.sub.p,ij be the
correspondent adjacency matrix, where a.sub.p,ij=1 means that the
i.sup.th device (node) can communicate with device (node) j using
rate p and a.sub.p,ij=0 means that they cannot. Obviously, A.sub.0
is equal to a matrix of all ones except for the diagonal terms,
since all devices (nodes) are assumed to be able to communicate at
the basic rate R.sub.0 to their neighbors. Assume that all devices
(nodes) overhear the control packet and packet header of the data
packet between devices (nodes). Therefore, they can collect
information to update the matrices A.sub.p whenever the opportunity
of a message exchange arises. Further assume that if two devices
(nodes) communicate directly, they always do so at the maximum
possible rate.
As mentioned above, rate adaptation is essential to maximize the
network layer performance metric. The goal is to pick the coding,
modulation and space-time coding schemes for each transmission. For
example, to maximize the MAC layer throughput, one could minimize
the transmission time of a packet of B bits:
.times. ##EQU00002## where R.sub.1 and R.sub.2 are the data rate
for the first hop and the second hop, respectively.
Note that the MAC layer overhead and the bandwidth used to resolve
contention are ignored here. In at least some embodiments
consistent with the present invention, cooperative transmissions
are employed only when cooperative transmissions take less time
than direct transmissions.
The effective rate for a cooperative transmission is:
.times. ##EQU00003## R.sub.C is called the effective rate for a
cooperative transmission since 1/R.sub.C is the time required to
send a information bit, without considering the MAC overhead. For
R-DSTC cooperation, a group of:
.times..noteq..times. ##EQU00004## cooperative devices (nodes),
capable of connecting with the source device at a rate greater or
equal to Rp, might be collectively able to support a higher rate
towards the same destination device than any of them can do
separately. Note that the performance does not only depend on the
number of relay devices, but also depends on the average channel
quality of the relay devices. Let q.sub.p,sd, be the channel
quality of i.sup.th relay of source s using rate p to destination
d, and let Q.sub.p,sd=[q.sub.p,sd,1, q.sub.p,sd,2, . . . ,
q.sub.p,sd,s], the rate supported towards station d by all the
N.sub.p,s relays, be a function f(R.sub.p, N.sub.p,s,
Q.sub.p,sd).
Overall, assuming that the cooperative stations adopt a
decode-and-forward strategy, the maximum rate the MAC can request
for the overall link is:
.times..times..times..times..times..function..function.
##EQU00005##
.sctn.4.3.2 Exemplary Signaling Protocols
FIG. 4 is a flow diagram of an exemplary MAC layer protocol 400
consistent with the present invention. The network nodes discover
node-to-node (e.g., source to each relay, each relay to
destination) channel conditions in the wireless network. (Block
410) Then, one or more of (a) relay devices, (b) modulation schemes
(source to relay, and/or relay to destination), and (c)
transmission rates (source to relay, and/or relay to destination)
are determined (e.g., by the source device) using the discovered
channel conditions. (Block 420) At least some of the determined
values (e.g., relay to destination modulation schemes, relay to
destination transmission rates, etc.) are then signaled to the
determined relay devices. (Block 430) Referring back to blocks 410
and 430, the discovery and signaling may use layer 2 (data link
layer) signaling.
There are two kinds of basic channel access schemes, contention
based access and centralized polling/scheduling based channel
access. MAC protocols consistent with the present invention may
work in both channel access schemes.
In a wireless network, it is costly to feed back the number of
relays and channel information on the fly for each packet. This is
particularly costly when there are multiple relay devices since the
time used for feedback of such information increases with the
number of relay devices. On the other hand, however, the
performance of R-DSTC depends on the number of relay devices and
the channels between the relay devices and the destination device.
The underlying space-time coding and modulation scheme have to be
identical among all participating relay devices for each packet
transmitted. This makes it impractical to let the relays make this
decision in distributed manner.
In at least some embodiments consistent with the present invention,
all potential source devices (nodes) may detect and measure the
average number of relay devices, and the average channel quality to
these relay devices. Each device (node) listens passively to the
packets sent by its neighbors and measures the received signal
strength. The received signal strength can then be used to estimate
the channel quality. The measurement is averaged over time so that
each device (node) knows of the statistical channel information to
all neighbors. By decoding the source/destination address and rate
information contained in the header of the overheard packets, each
device (node) can discover its two hop neighbors.
Alternatively, the one hop information used by a node can be
retrieved by using an active discovery procedure with neighbor
(peer) devices. The required bandwidth or channel time can be
either requested by the source device (e.g., a base station). All
relay devices send a pilot signal to the source device explicitly.
This is at the cost of wasting extra bandwidth. All the information
retrieved from the physical layer is then passed to the MAC layer
and stored in a database called the "Neighbor Table".
The performance of an R-DSTC consistent with the present invention
depends on the channel from the relay devices to the destination
device. Since this cannot be measured locally, the source device
will not be able to know such information unless it explicitly
exchanges the information stored in the "Neighbor Table" between
all devices (nodes). In this case, assume that the source device
knows only on average how many relay devices can receive at a
particular rate. By assuming that all relay devices are i.i.d
uniformly distributed in a circle of a radius identical to the
transmission range of the first hop rate, the average error
performance can be then calculated by averaging over all possible
relay device locations.
Based on the average error performance, the source device can then
decide on the second hop rate without knowing the location of the
relay devices. However, the performance is expected to be lower.
Such a MAC is called a "neighbor count-based" R-DSTC MAC. The rate
adaptation procedure should ensure that the desired loss rate can
be sustained (e.g., by averaging over the locations of the relay
devices).
A third rate adaptation algorithm is receiver feedback based, in
which the destination device receiver measures the received signal
quality. The destination device receiver then either sends a
command, or the quality of the signal to the source device. The
source device then adjusts the rate according to this command or
measurement.
The rate adaptation could also be a hybrid method of the above
mentioned methods. In a dense network, node density or neighbor
count is enough to exploit the full potential of the rate
adaptation; detailed channel information is not required.
Devices (nodes) can also exchange the stored information in the
"Neighbor Table" with their neighbors. This can be done either by
(A) periodically broadcasting the information using a control
packet, (B) piggybacking the information in a data packet, or (C)
exchanging the information on request by the source device (e.g.,
base station). Every device (node) in the network then knows the
identify of its neighbors, and the average channel quality to each
neighbor. The higher the frequency of information exchange, the
higher the throughput, though high frequency of information
exchange will increase signaling overhead. Therefore the frequency
of information exchange should be adapted to network mobility.
The neighbor count based R-DSTC MAC requires less signaling
overhead, but the performance is expected to be lower. The channel
information based MAC uses more time for signaling, but the
performance is expected to be better, since more information is
available. In a dense network, the difference in performance is
expected to be small and therefore channel information is no longer
necessary.
When a data packet arrives at the MAC layer from a higher layer,
the MAC layer inspects the "Neighbor Table". For each possible
modulation and coding rate in PHY, the source device can estimate
how many relay devices would be able to receive its transmitted
packet. Based on this number, the source device picks a size L for
the underlying STC. Ideally, L is close to the estimated number of
relay devices that can actually decode the packet from the source.
This ensures highest diversity (data rates) at the destination
device. Using the information in the "Neighbor Table", the source
device may also pick the coding and modulation schemes for use by
the relay devices so that the destination device can decode the
second hop transmission with high probability. By comparing all the
coding and modulation schemes for both hops, the scheme that
requires minimum time is used in the data packet transmission.
Since the transmission time for each packet is minimized, the
overall throughput for the network is maximized.
For every packet transmission, the source device may include in the
header the size of the underlying space-time code, and the coding
and modulation scheme needed for the second hop in the packet. Upon
receiving of the data packet from the source device, each relay
device may re-encode and modulate the received data according to
the parameters set by the source device.
In at least some embodiments consistent with the present invention,
the timing of the simultaneous transmissions from the relay devices
can follow after a fixed time interval, such as the short
inter-frame space ("SIFS") used in IEEE 802.11. Thus all relay
devices forward in synchronization. This is because the
transmissions from all participating relay devices have to be
synchronized both in time and frequency. The time accuracy of the
relay devices is less than the receiver's symbol duration time, and
within its capability to handle the delay spread of the system. In
a typical narrow band system, the symbol duration is larger than a
few microseconds. In a wide-band system, the current dominant
physical layer solution is orthogonal frequency-division
multiplexing ("OFDM"), whose symbol duration is also above a few
microseconds (10 us for IEEE 802.11g and 100 us for IEEE 802.16m).
One possible way to ensure synchronization in a manner consistent
with the present invention is to have the relay devices transmit
after a fixed time interval following packet receipt. As long as
the clock speed is larger than 10 MHz, it is not difficult to
guarantee accuracy to the order of 0.1 microsecond in hardware.
Another possible way to ensure synchronization in a manner
consistent with the present invention is to have all stations
synchronized to a reference clock, such as using a Global
Positioning System ("GPS") chip for example, and let the source
device clearly indicate the absolute time that all relay devices
should transmit.
In OFDM systems, frequency synchronization is also required. The
goal is to keep the drift of the central carrier frequency under a
certain threshold. This can be done in a manner consistent with the
present invention by allowing one device (node), such as the access
point, base station, the source device, or one of the relay
devices, to send a reference carrier signal with the relay device
oscillators locked to that reference carrier.
.sctn.4.4 Performance Assessment
Monte Carlo computer simulations were used to evaluate the
performance of an exemplary MAC scheme consistent with the claimed
invention. In the simulations, there is a base station (or more
generally, a source device) at the center of the network and mobile
stations (MS) are i.i.d located within a circle that has a radius
of 100 meters. All stations (or more generally, devices or nodes)
are equipped with one antenna and the modulation schemes supported
are BPSK, QPSK, 16-QAM, 64-QAM and 256-QAM. Considering practical
limitations, each station supports space-time processing up to L=6.
The underlying space-time code is assumed to be orthogonal. The
targeted error probability is 10.sup.-3. Symbol duration is equal
to Ts=10.sup.-7 seconds. The transmission power is such that BPSK
reach to the boundary of the network.
FIG. 5 illustrates simulations of how network throughput depends on
the number of relay devices under various transmission techniques.
FIG. 6 illustrates simulations of how service delay depends on the
number of relay devices under various transmission techniques. The
results indicate that in terms of throughput and delay, R-DSTC MAC
is much better compared to other MAC.
.sctn.4.5 Conclusions
Embodiments consistent with the present invention provide a method
for transmitting data from a source wireless device to a
destination wireless device, in which the source wireless device
first transmits the data to a group of relay devices, and then each
of the relay devices forward the data to the final destination
using a randomized space-time coding. The time required for the two
hop cooperative transmissions may be compared to direct
transmissions to determine whether or not the time for cooperative
transmission is less than direct non-cooperative transmission. In
some embodiments consistent with the present invention, if the
cooperative transmission time is determined to be less than the
direct non-cooperative transmission time, then the data is
transmitted from the source device to the destination device via
the intermediate relay devices; otherwise the data is transmitted
directly from the source device to the destination device.
Methods consistent with the present invention may be used both in
infrastructure-based networks, and in mobile ad hoc networks.
Methods consistent with the present invention may be used to reach
a destination device that cannot be reached using a direct
transmission from the source device.
* * * * *